Systems thinking, a framework for seeing the whole, is an effective means of dealing with real-world problems. This study explored the sources of systems thinking in school leadership. Qualitative data were collected via 82 semistructured interviews and 6 focus groups, among pre-service principals, novice principals and experienced principals. Data analysis included four stages: condensing, coding, categorizing, and theorizing. The analysis yielded four sources of systems thinking in school leadership: (1) managerial experience, (2) role model, (3) academic study, and (4) natural tendency. The findings expand limited existing knowledge about systems thinking in school leadership, and may assist in establishing ways to enhancing and accelerating the development of systems thinking among prospective and active school principals.